Gro-gro is essentially two words, not one, which are linked together by a magical hyphen. Gro-gro can be written with or without the hyphen, but using the hyphen ensures that saying 'gro' twice is not a typo, an indeed purposeful.
'Gro-gro' is © to Befera, because she made it up while at camp.
"Ew, it's a gro-gro!"
"That guy over there is such a gro-gro."
"Did you know that your mom is sort of a gro-gro?"
"That's pretty gro-gro."
"Why would you eat that? It's gro-gro!"
"That's probably the most gro-gro thing I've ever seen."